Multipath Rules using Additional Address

We have two Sophos UTM 210 in high availability.

We have two physical internet lines (call them Line-A & Line-B), and are using Multipath Rules to determine outbound packet flow between them.

Here is an example multipath rule (that I use for testing):

I have just added an additional logical address (call this Line-B.2) for a special purpose, as an Additional Address to the existing Interface for Line-B.  I would like to continue using Multipath, if possible, but instead of pointing to the physical interface, point to the logical "additional address".

The goal being to very easily be able to switch packets of a certain type to either go out the primary address on Line-B (call it Line-B.1) to the secondary "additional" address (Line-B.2).  Multipath Rules are awesome for this.

Is this possible?

If not, what is the substitute to achieve this?  Is there a graceful way to replicate this behavior using the Additional Address?  Do I have to physically wire up two additional patch cables to the UTMs & the internet line and create a new physical interface, just to replicate this behavior?

Thank you in advance!